Printing devices for printing a marking on a moving container are known in the art. Conventional print head assemblies usually comprise a motor so as to adjust the height position of the print head relatively to a conveyor so that containers of different heights may be processed.
For example, EP 0 534 337 A1 refers to a printing device for printing a marking on objects, which are continuously moved forward, in particular for parcels, wrapped magazine piles or the like. The printing device comprises a transport path alongside of which the objects can be transported at a defined rate of speed. By means of an adjustable matrix printing head, the surface to be printed of the objects can be printed in contactless manner when they pass the printing head. A scanning member, in particular an infrared distance sensor, is arranged at a distance before the printing head and scans the position of the surface to be printed of the objects and tracks the printing head according to the position of the surface to be printed of the objections by corresponding control over and adjustment device of the printing head.
Although the device of EP 0 534 337 A1 is suitable for adjusting the position of a print head, a situation may occur where the scanning member fails to detect an object or to correctly detect the height of such an object. Consequently, the object moved along the transport path will collide with the print head.
Another document is U.S. Pat. No. 7,434,902 B2 referring to a printheads and a system using printheads. The printing apparatus comprises a jetting assembly including a plurality of nozzles for ejecting droplets on a substrate moving relative to the jetting assembly and a mechanism for increasing the displacement of the jetting assembly relative to the substrate. Additionally, a sensor is provided, which includes a transmitter mounted on one side of the conveyor and a receiver mounted to an opposite side of the conveyor in a so-called “electric-eye” arrangement. If a substrate conveyed by the conveyor has a height that exceeds the spacing, the beam of light is interrupted and the transmitter sends a signal to a lift actuator so as to raise the mounting rack and the print head clusters above the substrate and also provides a signal to the print head clusters to interrupt the printing process.